1. Field of the Invention
This invention relates to a device for holding a sheet-like sample while reforming it flatly.
2. Description of the Prior Art
Generally, in an exposure device wherein the pattern of a mask is transferred to the surface of a sheet-like sample such as a semiconductor wafer, the wafer and the mask each is held by holders and positioned in opposed relationship with each other. The wafer, during repetitive exposure, is subjected to a process involving heat treatments such as development of photoresist and etching. Accordingly, each time it is subjected to the process steps, the wafer creates therein distortion, warping or the like with respect to its planarity. Such distortion or warping greatly affects the drawback of the chip on the wafer. It is therefore known to vacuum-attract the wafer to a sample supporting table on which the wafer rests, namely, a wafer holder, and forcibly reform the warping or the like and thereafter effect the exposure.
As an example of the conventional wafer holder which facilitates vacuum attraction of wafer, there is known one which comprises a flat plate formed in its surface of high planarity with a plurality of concentric circular grooves and a plurality of radial grooves connecting the circular grooves to one another and having an exhaust port opening into the interior of the grooves. A wafer placed on the surface of this flat plate is brought into intimate contact with and held by said surface when the air in the grooves has been exhausted through the exhaust port. However, even if the wafer is attracted and fixed over its entire area, if there is present foreign material such as dust or the like which has adhered to the surface of the holder or the reverse side of the wafer during the preceding step of process, the wafer attracted thereonto often creates therein minute warping under the influence of the foreign material, and deteriorated surface accuracy of the surface of the wafer results in reduced resolution of the projected pattern. Adherence of such foreign material can be prevented to a certain degree of reducing the area of contact between the surface of the holder and the reverse side of the wafer, but it is also important to form grooves of such a pattern so that the foreign material adhering to the contact surface can readily escape into the grooves.
On the other hand, in the conventional wafer holder, where a wafer of a small diameter is to be attracted, leakage of air through the grooves occurs in the outer peripheral portion of the wafer to make the attraction insufficient because the distance from the aforementioned exhaust port to the outer periphery of the wafer becomes smaller, and it has therefore been necessary to replace the holder in accordance with the diameter of the wafer.